1.4 Sea-Floor Spreading

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Vocabulary

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Mid-Ocean Ridge: The longest chain of mountains in the world.
Sonar: A device that bounces sound waves off underwater objects and then record the echoes of these waves.
Sea-floor spreading: The process that continually adds new material to the ocean floor.
Deep-ocean trenches: Deep underwater canyons.
Subduction:The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

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Outline

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Introduction

• The Ocean Floor
  • Freezing temperatures
  • Crushing pressure
  • Most bizarre creatures ever found.
    • Ex. Giant red tube worms, Giant clams nearly a meter across and strange spider-like crabs

Mapping the Mid-Ocean Ridge

• Mid-Ocean Ridge is the longest chain of mountains in the world.
  • Goes through all the oceans and around the world ( Only underwater )
    • Use sonar to locate it.
      • Sonar is a device that bounces sound waves off underwater objects and then records the echoes of these waves.

Evidence for Sea-floor Spreading

• At the mid-ocean ridge, molten material rises from the mantle and erupts spreading molten material and pushing older rock to both sides of the ridge.
  • Harry Hess, an American geologist, was one of the scientists who studied the mid-ocean ridge.
    • Hess called the process that continually adds new material to the ocean floor is called sea-floor spreading.

Evidence from Molten Material

• In the 1960's, scientists found evidence of new material.
  • Scientists found pillow like lava ( forms when magma cools very quickly) on the ocean floor.

Went down in a submersible named Alvin.

Evidence from Magnetic Stripes

• Scientists have found patterns in the rocks on the ocean floor.
  • Evidence shows that the Earth's poles have reversed themselves.
    • This last happened 780,000 years ago.
    • ocean floor lies in a pattern of magnetic stripes.
    • These stripes hold the record of reversals in the Earth's magnetic field.

Evidence from Drilling Samples

• The Glomar Challenger, a drilling ship built in 1968, sent drilling pipes through the water to dig deep holes in the ocean floor.
  • It sent the pipes 6 kilometers down.
  • Samples were brought up through the pipes.

Subduction at Deep-Ocean Trenches

• A deep ocean trench forms where the oceanic crust bends downward.
  • Suduction occurs at these places.
    • At deep ocean trenches, subduction allows part of the ocean floor to sink back into the mantle.

Subduction and Earth's Oceans

• The ocean floor is renewed every 200 million years.
  • This is the time it takes the rocks to form at the mid ocean ridge.

Subduction in the Pacific Ocean

• The Pacific Ocean covers one-third of our planet and it is shrinking.
  • The Pacific Ocean has a lot of trenches.
    • A deep ocean trench swallows more oceanic than the mid-ocean ridge can produce.
    • Then if the mid-ocean ridge does not add new crust fast enough it will shrink.
    • This is what is happening to the Pacific Ocean.

Subduction in the Atlantic Ocean

• The Atlantic Ocean is expanding.
  • Unlike the Pacific, the Atlantic only has a few short trenches.
    • As a result the new ocean floor has nowhere to go.
    • Therefore the Atlantic Ocean is expanding.